Means for and method of reducing interference



Jan. 6 1 92 6.

. 1,570,771 H. NYQUIST mums-Fora AND METHOD OF 'Rnbucme INTERFERENCE 4 Sheets-Sheet 5 MTORNEY Jan. 26 1926;

H; NYQUIST nuns FOR AND uz'rnon 011 aswci'ue m'rzam'aznca Filed Harh 2.0, 1922 4 Sheets-Sheet Z INVENTOR. Z11

Q k ax ATTORNEY Patented Jan. '26, 1926.

UNITED STATES P TENT" OFFICE.

HARRY NYQuIs'r', E EEMHURs'r, NEw YoRk,- Assm oE T0 AMERICAN TELEPHONE AND TELEGRAPH. COMPANY, A CORPOR ION OF NEw YORK.

MEANs on ANn MErMon or ammonia INTERFERENCE.

Applicationfiled arch '20, 1922 Serial No. 545,296.

To all whom it may conbem:

Be it known that I, HARRY NY UIs'r, resid- I ing at ElnihurSt, in the county of Queens and State of New York, have invented certain Improvements in Means for and Meth-. eds of Reducing Interference, ofwhich the following is a specification.

This invention relates .t o multiplex signaling, and more particularly to means for and methods of reducing interference between thechannelsof a multiplex signaling system.

Heretofore, in the operation of'multiplex carrier systems, it has been found that the repeater tubes employed atpoints along the line for transmlttmg the carrier frequencies,-

' near those of the interfering harmonics, etc.

Interference of this character becomes especially serious in a carrientelegraph system involving a large number of channels employing carrier frequencies within the usual voice range, for in such a case it isalmost essential to economic working of the system that the channels be narrow and closely. spaced, and that all the channelsbe amplified by the Same repeater tube.

In accordance with the-present invention, it is proposed to overcome this difiicultyby cloosing the carrier frequencies so. that they will bear a relation to each other such that the harmonics and sum and difference frequencies produced by modulatingthe several carriers with each other will fall in'a osition in the frequency spectrum substantially half-way between adjacent"carrier frequen-' cies. For example, the carrier frequencies may be made odd harmonicsof some funda-' mental frequency, say 125 cycles persecond. In this case, all sum and diiference frequencies and all second harmonics of the carrier frequemia Will. be even multiples of this I multiples of the fundamental.

fundamental frequency. It is therefore possible to design the selecting filters in such a way that they will transmit the major part of the band between two adjacent even multiples of the fundamental fre uency without transmitting the even multip es themselves.

Asa still further refinement of the invention, it is proposed to adjust the initial phase angles of the carriers ofthe various channels in such a manner as to reduce the interfering frequencies to relatively small amplitudes. I

, The various carrier frequencies involved may be generated independently without at tempting to maintain them in exact numerical relations. While this will substantially satisfy the first condition by bringing the interfering frequencies substantially midway between'the carriers, it will be apparent that even though the individual carriers be maintained of "constant and of invariable frequency, the. difierence frequencies and second harmonics will not all be the same if the carriers are not all exactly evenly spaced. As a result of this, the composite wave made up ofthe interfering' frequencies will vary inamplitude from instant to instant, even though the individual carrier frequencies from "which these components are derived arenot. va ying. It is ossible, however, to

maintain the relation etween the various carriers exact by derivin the carriers as harmonics'of the same undamental frequency. In this case, all of the-difference frequencies will bethesame, and the sum frequencies. difference frequencies and harmonics will, as previously stated, be exact When the carriers are generated. in this manner the phase relations between the different carrierstmay be controlled-or adjustedwith respect to each other to determine the amplitude of the disturbing frequencies just 'mn ti'oned; I

If, for example, the odd harmonics of some fundamental are to be used as carrier;

frequencies it will be seen that the initial phase relations of each of the carriers may be made such that when the fundamental wave is of maximum value and of a given sign all of the carrier waves will be ofmaximum value'at that instant, and of the same -sign as the fundamental. In order that'this condition may exist at any instant, it will be found that the carrier waves must bear to each othera definite phase relation so that for each complete cycle of the fundamental each of the carrier waves-will have a maximum positive value when the fundamental wave has its maximum positive value, and each carrier wave will also have a maximum negative value when the fundamental wave attains its maximum negative value. Also, each of the carrier waves will pass through zero when the fundamental wave .passes through zero which will, of course, occur twice for each cycle of the fundamental I have discovered by analyzing the interfering frequencies above referred to that the amplitudes of all of them will be greatest when the several carrier frequencies from which they are obtained bear the phase relations with respect to each other whlch have just been described, and by adjusting the phas'e'rel'ations of the individual carriers in various combinations to avoid these colncldences of phase I am able to reduce the amplitudes of the interfering. frequencies to quite small values.

The invention may now be more fully understood by reference to the following detailed description, when read in connection with the accompanying drawings, in which Figure 1 is a schematic diagram of a circuit arrangement embodying the principles of the present invention; Fig. 2 is a circuit arrangement showing the harmonic tor employed in the system of Fig. 1 in more detail; Fig. 3 is a diagram showinig1 the detailed arrangement of the type of lter employed in Fig. 1; Fig. 4.- is a series of curves showing the characteristics of the filters of a plurality of channels; trate in simplified form a phase changer which may be employed in .the circuit of Fig. 1-; Fig. 7 illustrates the circuit arrangement ofthe phase changer in greater detail,

,and Fig. 8 illustrates a method of determining how to change the phase angles of certain carriers in order to produce the best results.

Before discussing in detail the theory underlying the present invention, it will be well to outline the system to which the invention is applied, The invention is of particular utility when applied to a multiplexcarrier system having a large number of channels, each employing carrier'frequencies within the ordinary voice range, the individual channels being used for the transmission of telegraph signals. Referring to Fig. 1, L designates a transmission line upon which the several carrier channels may be superposed. 3 The carrier frequencies themselves are derived from afundamental source S, which may be, for example, a vacuum tube oscillator generating a fundamental frequency of say about 125 cycles 'per second.

genera- Figs. 5 and 6 illus-- The frequency generated by this source may be impressed upon a harmonic generator HG. This harmonic generator may be of any welleknown type of distorting arrangement, such as a distorting vacuum arrangement of which is shown in greater detail in Fig. 2.

Referring to Fig. 2, two vacuum tubes 10 and 11 are shown, having their input andoutput circuits connected in parallel. The grid circuit ofeach tube includes a resistance, as shown at 12 and 13, and the resistance in the grid circuit is so adjusted with respect to the plate and grid potentials as to produce a distortion of the wave impressed upon the tube in a manner which is well understood in the "art. The distortion of the wave impressed upon the input circuit of the tube produces harmonics of the impressed wave in the output circuit. By connecting the tubes'in parallel-as shown, and impressing the fundamental wave from the source S upon the two grids serially through the transformer 14, the even harmonics will be balanced out so far as the secondary of the balanced transformer 15 in the output circuit is concerned.

This action may be better understood from a brief consideration of the theory of a distorting vacuum tube. Assuming that 0 represents the voltage of the fundamental frequency and zflrepresents' the current in the output circuit of the tube, the current may then be expressed as an exponential series in terms of the impressed voltage, so that if the impressed voltage 6 is positive; the current in the output circuit will be If now-the same wave is impressed upon the other tube in opposite phase relation, as

is the case in Fig. 2, a will in this case be negative and the current i, in the output circuit of the tube ll. may be expressed i a be 0e (Z63 +fe, etc. (2)

t=2be+2de ,'etc. (3)- n It will be observed that all Of the even powers balance each other and only the odd quency of the fundamental, and that the expression. le corresponds to the third harmonic, and similarly, other odd powers correspond to other odd harmonics. Consequently, the arrangement of Fig. 2 will produce only odd harmonics. Y

Again referring to Fig. '1, it will be noted that the output side of the harmonic generator HG, whose operation has just been described, is connected to a pair of bus-bars 20, from which branches through'clrannels TL TL TL etc. are led; Eachchannel includes a suitable filtering arrangement, as indicated at F F F etc., which may be a simple tuned circuit sharply tuned to -the particular harmonic which is to be used as the carrier for that frequency.' The next element in each channel comprises a phase changer indicated at PC P0,, P6,, etc., the function of which is to adjust the phase relations of the carriers applied to the different channels with respect to' each other, so as to reduce the amplitudes of the interfering har- -monics and-sum and difference frequencies described hereafter;

in amanner which will be described later. The detailed construction pf, the phase changer is illustrated in Fig. 7 and will be The various frequencies assigned to the channels may :be-a'mplified .by amplifiers such {as A A A etc.., which may be adjusted to bring the generated? carrier frequencies to the same amplitude. These amplifiers may beof'any well known type, such for exam:

ple, as vacuum-tubeamplifiers; 3 The amplilineo The construction of the filters such as BF 5 fied frequencies may now be impressed upon a common line L over bus-bars 21 and throu h band filters such as BF BF BF etc. uitablev transmitting devices "T ,:T,, T etc, mayibej'arranged to interrupt the carrier frequency applied to the bamlffilters,

thereby causing signals to be transmitted by means of the carrier currents applied tothe etci,'is"illus trated in Fig. 3,.which shows a .ba-nd filterjof thefCampbell type, compriselements and shunt-anti-resonant elements. I11 order to give it as greatfan impedance as j which isconnected next to the bus-bars 21 is made afull series section.

ing two sections each having series resonant possible to...current's outside of the'range' which it :ismdesired to transmit, the section The other end ofthe, filter is connected to the transmitting side of tli'e circuit.

Fig.4 shows the transmission characterisetc, these being the successive odd'harmonics of a fundamental frequency of 125 cycles. It will be observed that the transmission loss for any one of these'filters over a range of about 200 cycles is less than 2-miles, while the loss for frequencies which are even multiples of 125 cycles is about 14 miles. If then the carrier frequencies are so spaced relatively to eachother asto produce interfering frequencies failing at points where the attenuation curves ofthe filters in Fig. 4 CYOSS each other, the interfering frequencies will be so greatly attenuated .as to produce but little eliect upon the adjacent channels. Let us assume, for example, a fundamental frequency N. The odd harmonics generated by the harmonic generator HG and selected into the various channels will then be 3N, 5N, 7N, etc. If these waves be impressed upon some device having a non-linear characteristic, such for example, as a vacuum tube amplifier in the line circuit, the several carrier frequencies will, tend to modulate with each other, thereby producingdisturbing frequencies which are principally due to the square term .andga're consequently made -up,' as is well known,*:of second harmonics of the carrier and sumand difference frceach pair of adjacent. carriers will be 2N;

the difference frequencies due .to 'the beating action between every other carrier will be 4N, etc. the beating action of the first and second carrier will be 8 N; that resulting from'the beating action of the first and-third carrier will be ION; that resulting from. the beating action of the secondandfthirdearrier will I frequencies-set up asa result of the distor- "tionare 2N, 4N, 6N, etc., all of whichcorrespond to even multiples ofthe fundamental fi'equeney NI Therefore, for a funda- The sum frequency resulting from be'ieN, etc. It will thus be seen that the j y mentalfrequeney of 125'-cycles, as was asfromthe distorting action of amplifiers and *the likeiwill all fall at the points of'gr'eatest :attenuation ofthefilter arrangement.

It is, of course, notabsolutely necessary that the frequencies be produced by .t he harmonic method, -as very good results may be.

obtained where thefspacing of the channels 'isymaintained approximately the same, but

"the best resultswill be obtained if the spacis absolutely aecurate,'-- as is the case herethe carrier frequencies are produced as harmonicsof a fundamental.

Where-the spacing of the'carrieiffrequencies is maintained constant and accurate, as is the'case where the carriers are harmonics of-a fun'dam'ental frequency,.it is possible to sumed in the case of the filters illustratedin .Fig. '4, the interfering, frequencies resulting 3 combine the carrier frequencies in such phase relation with respect to each other that the resulting distortion is reduced to a minimum. In order 7 tounderstand the manner in whichthe interfering frequencies may be controlled by controlling the phase relations ofthe several carriers it will be necessary to analyze the peculiar relations existing between a number ofwaves which are oddharmonies of a common fundamental. If we let N represent the fundamental frequency the composite wave resulting from the superposition of a number of odd harmonics u on the fundamental wave may be represente as, follows:

cos (5pt 0 etc. (4)

which p is Qn-N and t is the time. In the above equation the first term represents the fundamental, the second term the third harmimic, the third term the fifth harmonic, etc.

It willbe readily apparent that if the funwaves will pass through zero whenever the fundamental wave passes through zero, which Wlll occur twice during each cycle of the fundamental. In other words, there will be a coincidence of the cyclic conditions of all of the waves four times for each cycle of the fundamental. If, however, the initial phaseangle of one of the waves be shifted,

.this relation will be disturbed, and the four coincidences per cycle of the fundamental will not occur. This is for the reason that each time a coincidence occurs for, all of the waves except the one Whose phase has been shifted, the particular wave which has been shifted will have passed beyond the condi tion corresponding to the coincidence by an amount determined by the initial phase angle to whichthe wave was shifted. In order that the four coincidences above referred to may occur, therefore, it is necessary that the fundamental and all of the odd harmonics simultaneously start their cycles under conditions corresponding to one of the points of coincidence.

I have discovered by analyzing the inter fering frequencies, such as the sum and di fference frequencies" and harmonic terms resulting from intermodulation of the individual carriers, that these interfering frequencies will attain their maximum amplitude when the phase relations between the carriers are such that the coincidences above referred to Occur. By shifting the initial phase angles of certain of the carriers, the

components producing the interfering frequencies will, to some extent, tend to oppose each other, and, consequently by a proper adjustment of the initial phase angles, the components may be reduced to a I minimum. I

"While there is no general rule applicable to all cases, the manner in which the phase relations may be determined may, in general,

be best understood by reference to a specific application. Suppose we have a number of carrier waves corresponding to odd multiples of a fundamental wave, which may be represented as cos pt. The composite wave resulting from the nine carriers corresponding to the odd multiples from 3 to 19, inclusive, may be represented as follows:

cosllpt cos5pt cos7pt cos9pt cos11pt+ cos13pt cos 1519i cos17pt +cos19pt. (5)

.In the above expression, 37 may be taken to be, for example, 125 times 1r. Let us. now consider the following wave, which is equivalent to it except that a phase shift of 180 degrees has been introduced into some of the component waves.

cosSpt cos5pt +cos7pt cos9pt cos 11ptcos 13pt +lcos115pt cos 171a +cos19pt. (6)

The frequencies in the interfering range due to the square term and their magnitudes in arbitrary units in the two cases, are givenin the table below.

Frequency 750 Amplitude, first case; 8 76.5 6 5.5 5 4.5 4 3.5 4 Amplitude, secondcase 0 3 1.5 3 2.5 0 1.5 0

The manner in which the results set forth 1n the foregoing table may be obtained will be understood by referring to the chart shown in Fig. 8 of the drawing. In this upon the'diagonal line at the point where the vertical line drawn through each carrier frequency 1n the horlzontal row intersects the horizontal line drawn through the corresponding frequency in the vertical row. For

example, the second harmonic of the carrier designated 3 will be 6 and of the carrier designated 5, will be 10, etc.

Components, such as sum and difference.

frequencies resulting from the reaction of any two frequencies, may likewise be set While? in the horizontal row and'9 in the vertical row intersect at the leftof the said line. Either of these two points of intersection may be used for writing the difference frequency, and inaccordance with'the arbitrary [convention previously stated, the

difference frequency 2 will be written at the intersection on the right of the line, and

the corresponding sum frequency 16 will be written at the corresponding point of intersection on the left of the line.

Following this convention, it will be seen that a series of difference frequencies represented'by 2 and corresponding to the dif-' ferences between successive carriers, may be written on the first row of diagonal intersections to theright of the dotted line. An-

other diagonal row of' ls corresponding to the differences between alternate carrier frequencies will be set down in-the next row of diagonaljintersections, etc. .No frequency higher than 20 need be put down as frequencies higher than this frequency, will be so far above the highest carrier, which is 19, as to be of substantially no effect. When all of the sum, difference and harmonic frequencies which are not higher-than 20 are set down in the chart inaccordance wIth the plan given, the chart will contain the numbers indicated in Fig. 8.

,Suppose now all of the carrier frequencies are positive at a given instant, as will be the case under the condition represented by 1 the first algebraic expression above given. All of the numbers in the chart will tlien be positive. It willbe seenthat the same number occurs at different places in the chart. For example, all of the numbers corresponding to difference frequencies appear in the diagonalrow to the right of the diagonal line db, butthe same number may also appear in the diagonal l'ne and also at one .or more points to the left of the diagonal line.

If we take the number 6, for

. example, we see ,thatit occurs six times in the diagonal row to the right of the line ab and once on the diagonal line ab, but it does not occur to the left of the line al As this number occurs altogether seven times,

it would seem that the frequency repre sented by this number would have an amplitude seven times that of one of the difference components which makeup this frequency, assuming, of course, that all of the carriers involved have the same amplitude. It must be noted, however, that while correspondin sum and difference frequencies resulting from combining any two frequencies will each have the same amplitude, the

same rule will not apply to those numbers wh'ch represent harmonics.

For example, in the case of' the number 6 now being discussed, the wave represented by the number 6 at the top of the diagonal column to, the right of the dotted line ab 7 results from the beating action of two waves, one numbered 9 in the upper horizontal row and the other numbered 3 in the vertical row to the left of the square. The number 6 appearing on the dotted line,how-' ever, is a harmonic resulting from the energy of a single wave represented-by and not by the combined energy of two waves. Consequently, the amplitude of the harmonic terms appearing on the dotted line will only be one-half of the amplitude of a wave of the same" frequency appearing as either a sum or difference term. Thus the total wave inadeup of all of the- 6s in the chart will have an amplitude of 6% arbitrary units instead of 7, and assuming that 6 represents a single harmonic of a fundamental frequency of 125, or mother words, afrequency of 750, We find that the amplitude of this frequency will be 6.5 as set forth in the chart previously given.

Let us now considerrthe case of-the frequency represented by 8. This frequency occurs five times to theright of the-dotted line and once to the left of the dotted line,

butdoes not occur upon the dotted line itself. In other words, the frequency corresponding to 8 will be made up of five difference components and one sum component, and will, consequently, have an amplitude in arbitrary units of 6, as indicated in the chart above given, for the frequency of'lOOO cycles. Again, taking the frequency 14, we

find that'it occurs twice to theright of the dotted line, once upon the dotted line, and twice to the left of the dotted line, so that the corresponding Wave will have a total amplitude of 4%; arbitrary units, as indicated under frequency 1750 in the table. Again, in the case of the frequency represented by 16, we find that it occurs once to the, right a'ndthree times to the left, but does not occurupon the dotted line, so that the total amplitude of this frequency will Ob viously, these are the greatest amplibe tarbitra ry units, as indicated under the column representing 2000 cycles in the table.

tudes that any of these disturbing frequences may assume, and by changing the phase angle of certain of the carrier frequencies represented in'equation by 180 degrees, some ofthe individual components making up a given interfering frequency will cancel each other, so that the total amplitude of a given interfering frequency will I be reduced. Let us assume that the carrier frequencies represented by .9 and 13 in the chart of Fig. 8 are 180 degrees out of phase with the'other carrier frequencies when the other carrier frequencies, are in a condition at the intersection of a vertical line through .13 in the horizontal column and the horizontal line through 9 in the vertical column. The harmonics of 9 and 13, which would appear on the diagonal line ab, will also be positive, notwithstanding the fact that the wave from which they are derived is neg ative. This follows from the fact that the harmonic results from the square term. In short, if the carrier frequencies 9 and 13 are negative, all of the numbers in the vertical .columns :below'9 and 13 and the horizontal columns to the right of 9 and 13 will be negative, except those numbers occurring at points where the vertical and horizontal columns intersect each other.

Let us now determine the amplitude of'the waves representedby the numbers previously considered. The number 6, for example, which occurs 6% times, has four negative components corresponding to difference frequencies, so that the algebraic sum of the several components is 1 and the total amplitude of the wave of this frequency will I the column below frequency 7.50 in the table given above. The number 8 occurs four times positive and twice negative, so that the algebraic sum willbe 2,- and thus we have a positive amplitude of 2 units, asis indicated in the column" below 1000 cycles.

, The frequency l t-occurs 3% times positive and once negative, so that the total amplitude will be 2 units, as-indicated in' the table under 1750 cycles. The frequency 16 occurs four times, twice negative and twice positive, so that its amplitudewill be zero, as indicated in the column in the table under 2000 cycles. Comparing the amplitude in thetable for the two cases, we find that in the second case, where frequencies 9 and 13 are made negative, some of the interfering frequencies will be eliminated, and all of the others are materially reduced in amplitude. 1

While a. certain amount of cutting and trying is necessary in any given case todetermine which waves should be 180 degrees out of the phase relation corresponding to a condition of coincidence with the others, it will be more or less evident from a chart such as is shown in Fig. 8 that by making certain waves negative, a very large number of components of a given frequency will tend to cancel each other. .For example, the frequencies 9 and 13 in the chart of Fig. 8 will obviously have the desired effect when made negative by'reason of the fact that made by means of the phase adjusters shown in Fig. 1.

So far we have considered the results which will arise by 'Iarrangmg certain car riers to. be 180 degrees out'of the phase relation with the others which should exist at a time of coincidehce. A desirable reduction in' the interfering frequencies will result from other phase relations involvingangles other than 180 de rees, and in fact, any phase adjustment'wiich brings one or more -of the carriers out of the phase relation with others which produce the conditions of coin.

cidenceabov mentioned, will result in a diminution of the amplitude of the interfering frequencies. As,a practical proposition,

the best results in a given case might be ob tained, for example, by testing in the line circuit for interfering frequencies and by be 1% arbitrary units, as is indlcated 1n manually adjusting the phase changing de vices'in the several channels until such an adjustment is reached as will produce the least interference. I

This will,-of course, involve the use of a phase changing device which may be readily manipulated by hand, so that by turning a knob, for example, the current corresponding to a given frequency may be set out of phase with the electromotiveforce produc- 7 right and the movable switch 21 in its 8X?" phase of the current at 1 may be advanced with respect to an electromotive force applied at by including capacity in series with the line and inductance in shunt there-' with; With the switch thrown to the heme right position, the electromotivc force and current will bein phase with'each other, as there is'no shunt inductance, and the switch 21- being serially connectedin the line, the series capacity may be considered infinite. If the switch be moved one step-to the left, therewill be a large series capacity ahead of the electromotive force.

in the line and a large inductance inshunt with the line, so thatjthe current will be As the switch 21 is moved' still farther to the left,

- thefseries capacity and shunt inductance are decreased. step by step with continued advancement of the current with respect .to the electromotiveforce, until at the extreme left, position ofthe switch 21, a small series 1 capacity and a' small shunt inductance'will' he associated' with the line, having such values that the, current will be just 90 degrees ahead of theelectromotive force. ,By this arrangement, the phase of the current with respect to the electromotive force maybe adjusted throughout the first quadrant. Bythrowing the switch 20 to the left, the con-' nection to the line wire-s will be reversed, so

that for any setting of the'switch 21,- the current at I will be just 180 degrees out ofphase with the cu'rre'ntfor the corresponding setting-of the switch 21 when the switch 20 was thrown to the "right. With the switch 20 thrown to the left, therefore, any adjustment of the phase of the current with respect to the electromotive force may be made through the, third quadrant.

Fig. 6 shows an arrangement for adjusting the phase through the second and fourth quadrants. Considering the switch 20 in this instancethrown to the right, it will be seen that the switch 21 will cut in series inductance and shunt capacity. Such an arrangement, as is well known, operates as a phase retarder, that is, operates to' retard the phase of the current with respect to the electromotive force. With the switch 21 in its extreme left-hand position, it will be seen that all of the series inductance isshortcircuited and the shunt capacity .is opencircuited, so that the?"c.u1 'rent is in phase with the electromotive force, as was the case under the condition first described in con-- nection with Fig.5. Bymoving the switch 21 one step to the right, a small series induc-' tance and a small shunt capacity will-be 'con-' nected in circuit, thereby retarding the current with respect to the electromotive force by a small angle. By moving the switch still further to the. right, larger series inductance and larger shunt capacity will be connected in circuit, still further retarding the v current, until at theextreme right hand p.0-

sit'ion of the switch 21,.1arge series inductanee and a large shunt capacity of-such values as will retard the current 90 degrees behind the electromotive force are connected-i'n circuit. In other words,

by setting the sw tch 21 in'flits extreme right-hand position and moving it by successive steps to the left,

the current/may be shifted from W -degrees. behind the electromotive' force until It is exactly in phase with the electromotive' force, so that the phase angle between the current and th'e'electromotive :force may shift throughout the. fourthquadrant.

' Here again, bythrowing the, switch '20 to the: left, a relation will the produced hot-ween the current and lGGtI'OIIlOtlVGI force which is i 1st.18U degreks out ofphase with the corresponding relation when the switch is thrown to the right: This 'will enable any phase relationdesired-to be produced in the second quadrant; w v I {It now remainsto combine these four effects' so that by one continuous adjustment, the phase angle of the current with respect throughout 360 degrees 1 Referring to Fig". 7, a switching device is shown for connecting variousamounts of ments of this group of four is connected to. :one side of the. generator or other device for supplying an. alternating elec-tromot ive force, and the other pair of segments is conjnected to the other side of the source,

A. connection 39 extends from the switch member 33 to a terminal 40, to which a number of different capacities C to C inclusive, are connected; These capacities are connected to a number of commutation points or contacts, which may be engaged by arms 41 or -42 o'f'the switching device 26,. thesetwo arms being 180 degrees apart. A connection 43 extends from the arm'34 of the switching device 29 to a terminal 44 of a series of inductances L, to L inc1usive.

'These inductances are connected to commutation points or contacts to which-connections may be made by switch arms 45 or 46 of a switching device 27, these arms being 190 degrees apart. I

The arms-41 and 42"are electrically connected together, and produce the same result,.

I to the electromotive force. may be varied 3 one arm being operative during one'half rotationof the switch, and the other arm being operative during the other half rotation. Likewise the arms 45 and 46 are electrically connected together, so that one arm is operative during one-half rotation and the other arm during the other half rotation. The two switches 26' and 27, which comprise these four arms, are electrically connected together and are connected in common to the switch arm of the switch device 28. The switch arm 29 of the same switch is connected to the lower conductor 47 of the pair of conductors 47 and 48 connected tothc source.

The arrangement of the switching element 28 is such that when switch arms 29 and 30 rest upon sectors 31 and 32, respectively, line conductors 47 and 48 will be connected to line conductors 49 and 50, respectively. When, however, the switch arms 29 and 30 7 rest upon sectors 32 and 31, respectively, the

line connections are interchanged so that the current I will be 180 degrees out of phase with the current when the swit:.:h is in the former condition.-' The switching element 25, on the other hand, is so arranged that when switch arms 33 and 34 are on segments 35 and 38, respectively, capacity may be connected in series with the line wire 48 under the control of the switch 26, and inductance may be connected in shunt between the line wires 47' and 48 under the control of the switch 27 When switch arms 33 and 34 are i I arranged on sectors 36 and 35, respectively,

inductance may be connected in series with the line and capacity may be connected in shunttherewith. Withthe switch arms 33 and 34 on sectors 36 and 37, respectively, capacity may again be connected in series with the line and inductance in shunt therewith, while with these switch arms on seetors 37 and 36, inductance may be connected in series with the line and'capacity in shunt therewith.

With the various switches in the positions shown, the current I will be in phase with the electromotive force E, as a series circuit eliminated.

Suppose we now rotate the switch clockwise 90 degrees, which will correspond to the 'conditionof affairs at the beginning of the fourth quadrant of phase adjustment. Switch arms 46 and 42 of switching devi;es 27 and 26'willrest upon their seventh c0ntacts, and switch arm 33 of switching device 25 will be upon segment 38 and switch arm 34 uponv segment 37. A large inductance L, will now be in series with line wires 48 and 47 over a circuit from line wire 48, segment 37, switch arm 34, conductor 43, terminal 44, inductance L seventh contact of switch 27, switch arm 46, switch arm 30, segment 38 and sector 32 to conductor 50. 11' large capacity C will be shunted across the line from junction point 52, of the switching devices 26 and 27, over the arm 42 of switching device 26, seventh contact thereof, through the capacity C terminal 40, conductor 39, switch arm 33, segment 38 to line conductor 47. Under these conditions the current will be retarded 90 degrees behind the electromotive force.

If the switches now be moved counter clockwise one point, the conditionwill be the same, except that switch arms 42 and 46 rest upon their'eighthcontacts, so that a smaller inductance L; is connected in series and 'a smaller capacity (1,. is connected in shunt. by step in a counter-clockwise direction, the series inductance and shunt capacity are reduced step by step with a consequent reduction of the retardation of the current, until at the end of the fourth quadrant, switch arms 46 and 42 on their twelfth contacts. Under these conditions the shunt capacity As the switches are'advanced stepwill be open-circuited and all of the inductance eliminated. The direct series connection will be extended from conductor 50,

over switch arm 30, terminal 52, switch arm 46, twelfth contact of switch 27, terminal 44, condctor 43, switch arm 34, segment 37 and line wire 48. The current will now be in phase with the e'lectromotive force. The conditions above described correspond. to various adjustments throughout the fourth quadrant, so that the current will beanywhere from 90 degrees behind the electro.

motive force to exactly in phase with the electromotive force.

As the switches are stepped one additional step in a counter-cloclnvise direction, switch arms 46 and 42 rest upon their seventh segments and we have the condition previously described at the beginning of the first quadrant, with the current in phase with the electromotire' force. Upon stepping the switches one step further in a counter-clockwise direction, switch arms 45 and 41 rest upon the second contacts of switches 27 and 26, respectively, so that a large series capacityis connected in the-line from wire 39, terminal 40, capacity C second contact of switch 26, switch arm 41 to terminal 52. @A large inductance L is connected in shunt across the line between terminal 52 and line wire 47 from terminal 52, switch arm- 45,-second segment of switch 27, through inductance L terminal 44, conductor 43, switch arm34,

and segment 38, to line'wire 47. This will advance the current ahead of the electromotive force by a small angle, and for each successive step of ,the switch in a counterclockwise direction, a less series capacity and shunt inductance is included in the circuit with a corresponding advance in the phase of the current with respect to the electro motive force, until switch arms 41 and 45 rest upon the sixth contacts. In this case, a small capacity C, is connected in series and a small inductance L inshunt, these values being so chosen that the current will be just 90 degrees ahead of theelectromotive force.

When the switch arms 41 and 45 rest upon' the seventh contacts, we have the-same condition, so far as capacity and inductance are concerned as at the beginning of the fourth quadrant, namely, a large serieslnductance 5 serially connected between wires. 48-and 50, and a large shunt capacity C bridged across the line from terminal 52 to wire 47 through switch arm 33' and segment 36'. Switch arms 41 and" 45 will now-be effective 'result is the same.

to wipe over the contacts instead of switch arms 46 and-'42 .as described in connectionwith the fourth quadrant, but the eiectrical The circuit as a whole differs from the circuit condition existing as described in connection with the fourth quadrant, howeventin that switch arm 30 rangement of Fig. 7, it is possible to conof switch 28 now nests on sector 31 and switch arm 29 upon sector 32, thereby reversing the connectlons from the phase changer to the outgoing circuit 4950 Thisthrows the current 180-degrees outfof phase with the current existing'unde'r'the similar conditions described atthe beginning offthe "fourth quadrant, and consequently, the,cur-- es b hind the rent, instead of being 90 de electromotive ferce,-1s,- in e ect, 90 degrees ninth, tenth and eleventh contacts, the current is advanced over thequadrant between 90 and 180 degrees ahead of the electromotive force, until said switch arms rest on their twelfth contacts. I Under these circumstances, the shunt capacity is open-circuited' and the series inductance eliminated, so that,

remembering that wir 49..and 50 are now transposed, the current is. 180 degrees out of phase with the electromotive force;

The next movement-of the switch will bring switch arms 42 and .46 upon the first contacts of-switches26 and 27, and. switch arm 33 of switch ml rest upon segment 37 and switch arm 34 upon segment- 36.

We now again revert to the conditions of series capacity and shunt inductance, but the series capacity is elimlnated and the shunt inductance open-circuited. This would bring the current in phase with the electromotive force, except-for'the fact that switch 28 is still in such positionithat switch arm rests upon sector 31 and switch arm 29 upon sector 32, reversing the-outgoing conscribed inconnection with the first quad- 7 rant, except that a phase -diiference of 180 degrees comes in, because of the reversal of the connections of wires 49 and50. -'When wipers 42 and rest upon the seventh contacts, we will have a small series capacity C, and a small shunt inductance L This brings vthe current 90 degrees plus 180 degrees, or 270 degreesyahead' of the electromotive force, which is merely another way of saying that the current is'90 degrees behind the electromotive force, the condition which obtained. at. the beginning of the fourth quadrant. -When swltches 42 and 46 rest upon the seventh contacts, the same phase conditions will still exist, and the-connections are identicalwith those described at the beginning of the fourth quadrant.

It will be seen-that by means of the artinuo'u sly adjust the phase angle of the cur- I Consequently, by using hase sh fters of this character, or equivaentarrangements, in the circuit of ried until the interfering second harmonics and sum .and difference frequencies are reduced to the lowest magnitudes.

It will be understood that in' some cases it will be suflicient to merely employ the principle of so choosing the carr er frequencies rent with respect to the; electromotive force through 360 degrees.

Fig. '1, the phase relations of'the various Y carriers applied to the, channels may be valation of the phase relations of the; individual carriers. In other cases. it may be advantageous to employ both principles in combination, so thateither or bothof these expedients are within thescope of the'present invention."

It will also be obvious that the general rinciples herein disclosed may beembodied in many other organizations widelydifierent from those illustrated, without depart ing from the s irit of the invention-as defined in the 0 owing claims. .i

What is claimed is:' i 1. In a multiplex signaling system employing difierent carrier frequencies for a as to cause the interfering frequencies to fall plurality of channels, the method of reducrelated to each other that the sum and difference frequencies resulting from intermodulation of the carrier frequencies will fall substantially midway between adjacent carrier frequencies.

3.111 a multiplex signaling system employing difierent' carrier frequencies for a plurality of channels, themethod of reducing interference between'the channels due to interaction between frequencies of different channels, which consists in generating carrier frequencies for the various channels so related to each other that the second har monies and sum and difierence frequencies resulting from intermodulation of the carrier frequencies will fall substantially midway between adjacent carrier frequencies.

4. In a multiplex signaling system employing different carrier frequencies for a plurality of channels, the method of reducing interference between the channels due to interaction between frequencies of different channels, which consists in generating carrier frequencies for the different channels so related to each other that they will be odd harmonics of some one fundamental frequency.

5. In a multiplex signaling system, a plurality of transmission channels, means for supplying different carrier frequencies to each channel, and means to so relate the frequencies of the channels that the second harmonies of the various carrier frequencies will fall substantially midway between adjacent carrier frequencies, thereby reducing interference due to interaction between the frequencies of different channels.

6. In a multiplex signaling system, a plurality of transmission channels, means for supplying different carrier frequencies to each channel, and means to so relate the frequencies of the channels that the sum and difference frequencies obtained by intermodulation of the various carrier frequencies will fall substantially midway between adjacent carrier frequencies, thereby\ reducing interference due to interaction between the frequencies of different channels.

7. In a multiplex signaling system, a plurality of transmission channels, means for supplying diflerent carrier frequencies to each channel, and means to so relate the frequencies of the channels that the second harmonics of the carriers and sum and difference frequencies obtained by intermodulation of the carrier frequencies will fall substantially midway between adjacent carrier frequencies, thereby reducing interference due to interaction between the frequencies of different channels.

8. In a multiplex signalingsystem, a plurality of transmission channels, means for supplying different carrier frequencies to each channel, and means to so relate the frequencies of the channels that they will bear the relation to each other of odd harmonics of one common fundamental 1 frequency, thereby reducing interference due to interaction between the frequencies of different channels.

9. In a multiplex signaling system employing carrier frequencies bearing the relation of harmonics of a common fundamental for a plurality of channels, the method of reducing interference due to the square term components resulting from intermodulation of the individual carrier frequencies, which consists in impressing the several carriers upon the transmission circuit in such phase relation with each'other that a coincidence of phase conditions will-occur in less than all of the channels.

10. In a multiplex signaling system em- 6 ploying carrier frequencies bearing the relation of harmonics of a common fundamental for a plurality of channels, the method of reducing interference due to the square term components resulting from intermodulation of the individual carrier frequencies, which consists in impressing certain of the carrier frequences upon a common transmission circuit 180 degrees out of phase with the phase relation resulting in phase coincidence of all of the carrier frequencies.

11. In a multiplex signaling system employing carrier frequencies bearing the relation of harmonics of acommon fundamental.

for a plurality of channels, the method of reducing interference due to the square term components resulting from intermodulation of the individual carrier frequencies, which consists in individually adjusting the initial phase angles of the carrier frequencies relative to each other, so that the component electromotive forces making up the different interfering frequencies will at least partial- 1y oppose each other.

12. In a multiplex signaling system having a plurality of transmission channels, means for supplying carrier frequencies to each channel, said carrier frequencies bean ing the relation of harmonics of a common fundamental, and means for impressing the carrier frequencies of the difierent channels I ing from the square term component due to phase relation with each other that a coin cidence of phase conditions will occur in less'than all of the channels, whereby interfering components due to the square term resulting from dividual carriers. will bejreducedin' magnitude.

. 13. In a multiplex signaling system having' a plurality of transmission channels, means for supplying carrier frequencies to each channel, said carrier frequencies-bearing the relation. ofharmonics of a common fundamental, and means for impressing the carrier frequency from each channel upon a common transmission circuit in such phase relation that certain of the carrier frequencies will be 180 degrees out of phase with the phase relation resulting in base coincidence of all of the carrier requencies, whereby the interfering frequencies resultintermodulation of the individual carrier frequencies willv be reduced in magnitude.

14. In a multiplex system having a plurality of transmission channels, means for frequencies to each chancarrier frequencies bearin the nel, sai

nda-

relation of harmonics of a common mental, and means 'for impressing the,

various carrier frequencies from the several channels upon a common transmission circuit, said'last mentioned means including devices for adjusting the relative phases of the individual carrier frequencies, so that the individual components making up the interfering electromotive forces arising from the square term due to intermodulation of the individual carrier frequencies with each other will at least partially oppose one another. I

15. In a multiplex signaling system employing different carrier frequencies for a lurality of channels, the method of reduc mg interference between channels, which consists'm generating earner frequencles for a the various channels so related to each other ly midway between adjacent' carrier quencies, and in impress ng the CiLI'I'181 f1ethat the second harmonics of the carrier frequencies and sum and difference frequencies resulting from intermodulation of the carrier frequencies will fall substantfiglquencies so related upon a-transmission circuit 1n such haserelation w1th each other that a coinci ence'of phase condition will occur in less than. all of the channels.

' consists in generating carrier, frequencies 16. In a multiplex signaling system em ploying difierent carrier frequencies for a plurality of channels, the method of reducing interference between channels, which for the variouschannels so related to each other that the second harmonics of the carrier frequencies and sum and difference freintermodulation of the in qiliencies resulting from intermodulationof t e carrier frequencies will fall substantially midway between adjacent carrier frequencies, and in impressing certain of the carrier frequencies thus related upon a common transmission circuit 180 degrees out of phasewith the phase relation: resulting-in phase coincidence. of all of the carrier fre quencies.

17. In a multiplex, signaling system-employing different carrier frequencies for a plurality of channels, the method of reducing interference between channels, which consists, in generating carrier frequencies for the various channels so relatedto each other that thesecond harmonics of the carrier frequencies and sum and difference frequencies resulting from intermodulation of the carrier frequencies will fall substantially midway between adjacent carrier frequencies, and in individually adjusting the phase angles of the carrier frequencies relative to each other so that the component electromotive forces making up the different interfering frequencies will at least partially oppose each other.

18.-In a multlplex signaling system employing different carrier frequencies for a plurality of channels, the method of reduc ing interference between channels, which consists in generating carrier frequencies for the different channelsv so related to each other that they will be odd harmonics of some fundamental frequency, and in impressm the carrier frequencies of the v different-c annels upon a common transmlssion circuit in such phase relationwith respect to each other that interfering com- ,ponents'due to the square term resulting from intermodulation of the individua carriers will be reduced in magnitude. 1

19. In a multiplex signaling system, a,

plurality of transmission channels, means for supply ng different carrier frequencies to each channel, the frequencies of the chan- "nels being so related that the second harmonics of the carriers and and difierence frequencies resulting from intermodula'tion of the carrier frequencies will fall substantially midway between adjacent carrier frequencies, and means for impressing the carrier frequencie's'of the different channels upon a eommontransmission'circuit in such phase f'el tion with each other that a coincidence of phase conditions will occur in lessthan all of the channels, whereby interfering components due to the square term resulting from intermodiilation of the inv dividual carriers will be reduced in magnitude.

20. In a multiplex signaling system, a plurality of transmission channels, means for supplying different carrier frequenciesto each channel, the frequencies of the channels being so related that the seeond harmonics of the carriers and sum and difference frequencies resultingfrom intermodulation of the carrier frequencies will fall substantially midway between adjacent carrier frequencies, and means for impressing the carrier'frequencies from the several channels upon a common transmission circuit in such phase relation that certain of the. carrier frequencies will be 180 degrees out of phase with the phase relation producing phase coincidence of all of the carrier frequencies, whereby the interfering frequencies resulting from the square term'component due to intermodulation of the individual carrier frequencies will be reduced in magnitude.

21. In a multiplex signaling system, a plurality of transmission channels, means for supplying different carrier frequencies to each channel, the frequencies of the channels being so related that the: second harmonies of the carriers and sum and difference frequencies resulting from intermodulation of the carrier frequencies will fall substarr t-ially' midway between adjacent carrier frequencies, and means for impressing the various carrier frequencies from the several channels upon a common transmission ciraegan cuit, said last mentioned means including devices for adjusting the relative phases of .tion of the individual carrier frequencies with each other will at least partially op pose one another.

22 In a multiplex signaling system, a plurality of transmission channels, meansfor supplying different carrier frequencies to each channel, the frequencies of the channels being so related that they will bear the relation to each other of odd harmonics of a common fundamentalfrequency, and means for impressing the carrier frequencies of the differentchannels' upon a common transmission circuit, in such phase relation to each other that interfering components due to the square term resulting'from intermodulation of the individual carriers will'be reduced in magnitude. v

In testimony whereof,-I have signed'my name to this specification "this 18th day of March 1922. i

HARRY NYQUIST. o 

